Method of papermaking having zero liquid discharge

ABSTRACT

A method of papermaking having zero liquid discharge. A pulp slurry is produced from recycled cellulosic material and the slurry contains suspended cellulosic fiber material, solid contaminants and dissolved water soluble solids. The pulp slurry is subjected to a cleaning operation to produce a first flow stream containing the cellulosic fiber material and dissolved solids and a second flow stream containing dissolved solids along with solid contaminants. The first flow stream is washed with water to remove a substantial portion of the dissolved solids and provide a washed pulp which is used in a papermaking machine to produce a paper sheet. The second flow stream from the cleaning operation is compacted to produce a solid residue that can be landfilled and a liquid residue containing dissolved solids that can be recycled to the pulp cleaning operation. In a modified form of the invention, the liquid residue from the compaction can be subjected to a process, such as evaporation, to provide a solid residue and a purified water or condensate which can be used in the papermaking machine.

This application is a continuation-in-part of application Ser. No.08/654,248, filed May 28, 1996 now U.S. Pat. No. 5,762,758, which inturn is a continuation of application Ser. No. 08/298,748, filed Aug.31, 1994, now abandoned.

BACKGROUND OF THE INVENTION

Most paper or paperboard operations discharge tremendous quantities ofliquid which contain suspended solids, such as wood fibers, clay,calcium carbonate, titanium dioxide, sand, and wood bark, along withoxygen demanding organic compounds, such as starches, lignins, andhemicellulose. In addition, the liquid discharge can also contain tracechemicals, such as chlorinated organic compounds, polychlorinatedbiphenols, metals such as barium, aluminum, and other trace elements.The adverse environmental impact of this liquid discharge from apapermaking operation is well documented.

There are several reasons for the liquid discharge from a papermakingoperation. In some cases, the liquid discharge is due to careless,excessive use of water and, if excess water is used, it must bedischarged from the operation. In other cases, the liquid discharge isthe result of poor water system design, which results in inadvertent useof fresh water. In still other instances, excess water has been employedin an attempt to keep the suspended and dissolved solid levels at a lowenough level to ensure optimum paper machine conditions. In this lattercase, the added water effects a reduction in suspended and dissolvedsolids, but produces a corresponding increase in effluent flow.

Typically the most highly contaminated water discharged from aconventional papermaking operation utilizing recycled waste paper,results from the cleaning of the recycled pulp. The liquid residue fromthe cleaning operation is normally subjected to floatationclarification, in which the lighter weight solid material is recoveredas a floating layer which can be burned or landfilled. The liquideffluent from the clarification process, which contains dissolved solidsand oxygen demanding compounds, then must be treated by either anaerobic or anaerobic process before it can be discharged to the sewersystem. Evaporation of the liquid effluent has not been feasible due tothe tremendous volume of effluent.

SUMMARY OF THE INVENTION

The invention is directed to a papermaking operation having zero liquiddischarge. In accordance with the invention, a recycled cellulosic pulpis initially subjected to a cleaning operation. The cleaning operationin itself is conventional, and normally includes a number of stages inwhich various types of suspended solids or particulate material areremoved from the pulp. In a typical cleaning operation, debris and heavymaterial, such as stones, metal, glass, and the like are initiallyremoved from the pulp by a liquid cyclone, and the pulp is thensubjected to a coarse screening, followed by a secondary finer screeningto remove large and small size contaminants. Following this, the pulp issubjected to a forward cleaning operation utilizing a liquid cyclone toremove sand, small fiber bundles, ink, and the like. Several stages offorward cleaning are normally employed. The sand, dirt, and other solidmaterial that is removed during the forward cleaning is then subjectedto compaction, and the liquid phase from the compaction is discharged toa clarifier.

After the forward cleaning, the pulp is then passed through a slottedscreen cleaner, which removes smaller particles of plastics, hot melt,adhesives, and the like. The solid residue from the slotted screencleaner is again compacted and liquid removed during the compaction issent to the clarifier.

Following the slotted screen cleaning the pulp is subjected to reversecleaning utilizing a liquid cyclone, in which lighter material, such aswax, latex, hot melt, is separated from the heavy material, which is thepulp. Again, the reverse cleaning is normally carried out in a number ofstages, and the separated light phase is sent to the clarifier.

In the clarifier, which is preferably a floatation clarifier, the solidmaterials are separated as a floating upper layer or sludge which iscombined with the slotted screen and forward cleaner rejects and isdischarged to a mechanical solid waste dewatering unit, such as a sludgepress. A portion of the liquid effluent from the clarifier or from thesludge press may be sent to an evaporator.

In the evaporator, the liquid effluent from the clarifier or sludgepress is heated by a low pressure steam discharged from the dryersection of the papermaking machine. High pressure steam, at a pressurenormally about 160 psig, from a steam generating plant is supplied tothe dryers in the dryer section of the papermaking machine and a mixtureof low pressure steam at a pressure of perhaps 50 psig, and steamcondensate, are discharged from the dryers to a condensate tank wherethe low pressure steam is separated from the condensate. The lowpressure steam is then delivered to the evaporator and used as the bestsource to evaporate the liquid effluent from the clarifier. Thecondensate from the dryer steam as well as the condensate from thevaporized liquid effluent can be combined and delivered to a waterstorage tank, or alternately the condensate from the dryer steam, whichis relatively pure, can be recycled to the steam generating plant. Theevaporated residue from the evaporator is transferred to the solid wastedewatering unit where it is combined with the sludge from the clarifier.The combined residues are dewatered in the sludge press of thedewatering unit to produce a residue containing about 30 to 50% solids,and this material can then either be landfilled or incinerated. Theevaporator residue may be concentrated to the point that it iscrystallized or alternately the concentration of the dissolved solids iselevated to such a level that the sludge press reaches an equilibriumlevel where the pressed material carries with it the dissolved solids atthe rate at which they are entering the system.

The recycled pulp, after cleaning, is subjected to counter-currentwashing. Typically, recycled pulp is not washed in a counter-currentsystem. The counter-current washer uses very little wash water and actsto remove the majority of the dissolved solids in the pulp. A properlydesigned washer with the appropriately segregated water system, willserve as a barrier keeping high levels of ionic materials, such as alum;other soluble materials, such as starches; and colloidal material, suchas fiber debris, calcium carbonate and clay, from getting into the papermachine water system.

Following the washing, the pulp undergoes normal stock preparation andcan then be utilized in the papermaking process.

The condensed dryer steam and the condensate resulting from theevaporation of the liquid effluent from the clarifier can be combinedwith fresh makeup water in the water storage tank or chest, and thiswater is then employed in the papermaking process to be used in portionsof the process that require relatively pure water with a minimum mineralcontent, such as for example, the showers and pump sealing water. Waterrecovered from the papermaking process is recycled back to the pulpwashing operation and used to wash the pulp, and similarly, waterrecovered from the washing operation, can be recycled back and employedin pulp cleaning and pulping.

With the process of the invention, there is zero liquid discharge fromthe papermaking operation, the only discharge being solid residue thatcan be either landfilled or incinerated.

As a further advantage, the process of the invention produces a higherfiber yield, as no small fibers or fines are lost in water discharge.

The process of the invention also requires less energy than conventionalpapermaking processes. In a conventional process, water at an elevatedtemperature of approximately of 120° to 160°, is used as the incomingwater in the papermaking operation, and the water is normally heated tothis temperature either through direct heating or sparging with steam.In the process of the invention, the only required heat source is theheat needed for evaporation of the liquid effluent from the clarifier,and the steam from that evaporation will meet the heat requirements forthe incoming water.

Further, the process of the invention saves the energy normally requiredin the aerobic or anaerobic treatment of the liquid being discharged tothe sewer system.

As a further advantage, the process of the invention results in aconsiderably lesser water consumption than normal papermakingoperations. This is an important factor when the papermaking mill islocated in an arid region of the country.

In a modified form of the invention, recycled cellulosic pulp slurryfrom the pulping operation is subjected to a conventional cleaningoperation including the removal of heavy contaminants by centrifugalcleaners, followed by coarse and fine screening, forward and reversecleaning, and the separation of lighter phases through floatationclarification. The cleaning operation produces a first aqueous flowstream which contains dissolved solids and the majority of thecellulosic fiber material and is substantially free of solidcontaminants, and a second reject flow stream or sludge that consists ofhigh levels of solid contaminants and dissolved solids.

The cleaned cellulosic pulp in the first flow stream is then subjectedto countercurrent washing which serves to remove the majority of thedissolved solids in the pulp slurry. Following the washing, the pulpslurry undergoes normal stock preparation and then is utilized in apapermaking machine to produce a paper sheet.

The second or reject flow stream from the cleaning operation andconsisting predominantly of the solid contaminants is subjected to amechanical dewatering operation to produce a solid cake which then canbe discharged to a disposal site, such as a landfill or incinerator. Thewater effluent from the dewatering operation can be recycled and used inthe pulp cleaning operation.

As in the first embodiment, water recovered from the papermakingmachine, which can consist of water recovered from the showers and pulpseal water, as well as water extracted from the wet paper web, is thenrecycled to the pulp washer and used as wash water for washing the pulp.The water recovered from the pulp washing operation, consisting ofresidual wash water, as well as water recovered by virtue of theincrease in consistency of the pulp as it passes through the washingoperation, is then recycled to the pulping vessel for use in producingthe pulp slurry. A portion of the recirculated water from the washingoperation can also be recirculated to the pulp cleaner.

A primary concern in any attempt to develop a zero liquid dischargesystem, is the progressive buildup of dissolved solids in therecirculated water. However, with the method of the invention asubstantial portion of the dissolved solids are carried away in thesolid sludges or residue which can be landfilled or incinerated, thuspreventing an undesirable buildup of dissolved solids in therecirculated water.

The modified form of the invention does not require additionaloperations, such as evaporation, reverse osmosis or ultrafiltration, toremove dissolved solids from the water stream, but these additionaloperations can enhance the overall system by providing more purifiedwater for the papermaking operation, thereby increasing the availablewater for washing.

Other objects and advantages will appear in the course of the followingdescription.

DESCRIPTION OF THE DRAWINGS

The drawing illustrates the best mode presently contemplated of carryingout the invention.

In the drawings:

FIG. 1 is a flow diagram illustrating the process of the invention.

FIG. 2 is a flow diagram illustrating a modified form of the invention.

DESCRIPTION OF THE ILLUSTRATES EMBODIMENT

FIG. 1 is a flow diagram illustrating the papermaking process of theinvention having zero liquid discharge. A recycled pulp slurry isinitially produced in a pulping vessel 1 using conventional techniques.The recycled material can include used corrugated containers,wastepaper, white office waste, or the like, and can contain debris andsolid contaminates and also include a substantial quantity of fillers,such as clay and calcium carbonate, as well as ink, which normally wouldnot be present in a virgin pulp.

In the pulping operation, a caustic material such as sodium hydroxide,is added to the pulp, along with water, and conventional surfactants towet the fibers. Sodium silicate can be used as a deinking aid andhydrogen peroxide as a brightener.

In the pulping vessel 1, the pulp is maintained at a temperaturegenerally in the range of 120° F. to 160° F., and is subjected toagitation or a shearing action to break down the fiber bundles.

The pulping can either be a continuous operation, in which materials arecontinuously fed and discharged from the pulper, or a batch pulpingoperation. With a batch operation the pulp is maintained in the pulperat the above-noted temperature and with agitation for a period generallyin the range of 15 to 35 minutes.

The consistency of the pulp in the pulping operation can vary dependingupon the type of paper being produced. When producing corrugated mediumfor use in corrugated cartons, the pulp may be at a consistency in therange of 3% to 5% by weight of solids. On the other hand, when producingmore sophisticated grades of paper, such white office paper, the pulpmay have a consistency of 10% to 15% by weight of solids.

After pulping, the pulp is discharged to a pulp chest 2. With abatch-type pulping operation, the pulp chest serves as a storagefacility for the pulp, and the pulp is then discharged continuously fromthe pulp chest through the remainder of the papermaking process. When acontinuous pulping process is employed, the pulp chest will serve tocompensate for variations in flow from the pulper 1 and provide aconsistent flow throughout the papermaking process.

The pulp is then pumped from the dump chest 2 to a pulp cleaner 3, whichacts to remove suspended solids and particulate material from the pulp.The cleaning operation is, in itself, conventional and consists of aseries of sequential cleaning procedures. More particularly, large andheavy debris, such as bark, stones, glass, metal pieces, and the like,are initially removed from the pulp slurry, preferably by a liquidcyclone. The pulp then undergoes a coarse screening operation to removelarger-sized contaminants. Following the coarse screening, the pulp issubjected to a finer secondary screening, in which the pulp is subjectedto a shearing action, and smaller sized solids or contaminants areremoved.

After the secondary screening, the pulp slurry then undergoes a forwardcleaning to remove small particles of sand, fiber bundles, ink, and thelike. At this stage the pulp generally has a consistency of about 1.5%solids, and a liquid cyclone is employed for the forward cleaning, withseveral stages being utilized. The solid particulate material removedfrom the pulp during the forward cleaning is then transferred to acompactor and the liquid resulting from the compaction process isdischarged to a clarifier 4.

Following the forward cleaning the pulp slurry is passed through aslotted screen cleaner which acts to remove plastics, hot meltadhesives, and the like from the slurry. The residue from this cleaningoperation is compacted and the liquid recovered from the compactionoperation again is transferred to the clarifier 4.

At this stage the pulp may still contains small quantities of wax,latex, hot melt adhesives, and the like, and the pulp is then subjectedto a reverse cleaning operation in a liquid cyclone. In this operation,the lighter weight materials, such as wax, latex, and the like, areremoved from the heavier pulp, and again the reverse cleaning can becarried out in a plurality of stages. The lighter phase, which isremoved during the reverse cleaning, is also transferred to theclarifier 4.

The clarifier, in itself, is a conventional type and is preferably afloatation clarifier. The upper floating portion of the material in theclarifier is in the form of a sludge and has a solids content in therange of about 1.5% to 2.5%. The sludge is conveyed to a solid wastedewatering mechanism 5, while the liquid effluent from the clarifier 4is transferred to an evaporator 6.

In a typical papermaking process, steam from a steam generating plant issupplied to the dryers in the dryer section of the papermaking machine.The typical papermaking machine may contain approximately twenty to onehundred dryers and the paper web supported by a dryer fabric is passedover the dryers to dry the paper. It is customary to use a cascadingsteam system in the dryer section of the papermaking machine, in whichhigh pressure steam at a pressure of about 160 psig, is supplied to afirst group of dryers and lower pressure steam discharged from the firstgroup of dryers is supplied to a second group, of dryers. This cascadingsystem is continued, so that low pressure steam at perhaps a pressure of50 psig, along with steam condensate, is discharged from a last group ofdryers in the section and either condensed and returned to the steamgenerating plant, or discharged to the atmosphere.

In accordance with a feature of the invention, low pressure steamdischarged from the dryer section of the papermaking machine 9 isutilized as the power source to evaporate the liquid effluent from theclarifier. In this regard, the steam, at a pressure of perhaps 50 psig,and condensate from the dryers is discharged to a condensate tank wherethe steam is separated from the condensate. The condensate can then beemployed as a heat source to heat incoming process water to thepapermaking machine. The low pressure steam from the condensate tank, atperhaps a pressure of 45 psig, is supplied to evaporator 6 to evaporatethe liquid effluent. The evaporation is preferably carried out by amultiple effect evaporator, such as that sold by Goslin-Birmingham ofBirmingham, Ala. The low pressure steam from the dryer section isutilized to heat the liquid effluent to produce water vapor and organicvapors, due to the fact that the effluent may contain small amounts ofvolatile organic constituents. The vapor resulting from the evaporationof the liquid effluent is then condensed and transferred to a waterstorage tank 7. The condensed dryer steam from evaporator 6 can becombined with the condensate from the vaporized liquid effluent in tank7, or alternately, as the condensate from the dryer steam is relativelypure, it can be recycled to the steam generating plant, as shown in theflow diagram.

While a typical papermaking operation utilizes a cascading steam systemin the dryer section, a non-cascading system is preferred in theinvention, with high pressure steam being supplied to all of the dryersin the dryer section. The reason this is preferred is that all dryersmay be maintained at maximum pressure, which maximizes the drying rateand therefore minimizes the investment in drying equipment. Anon-cascading system is economically feasible with the process of theinvention because the steam being discharged from the dryer section issupplied to the evaporator 6 and used as a heat source for theevaporation of the liquid effluent from the clarifier.

The evaporated residue of crystals, if an evaporation is employed, mayhave a solids content of 40% to 60% by weight. It is possible, andpreferable, to not concentrate the residue to these high levels, sincethis may result in evaporator fouling. It is only necessary to evaporatea portion of the contaminated water in the processing loop, upstream orahead of the pulp washer, so as to supply the necessary quantity ofcondensate critical end uses, such as pump seals, etc. The solidscontent in the water system upstream of the pulp washing will rise to anequilibrium at which point the removals with the sludge will be equal tothe rate of addition with the wastepaper.

In the dewatering unit 5, the solid waste is dewatered by mechanicalequipment, such as a compactor, screw press, or belt press and the smallamount of liquid removed from the solid waste can be returned to theclarifier. The dewatered sludge having a consistency generally in therange of about 35% to 50% by weight solids, can then either belandfilled or incinerated. With proper design of the dewatering unit 5,the colloidal material will be entrapped by the filter cake and theionic solution in the filter cake increases as the lower ionic solutionis displaced. The incineration of the dewatered residue can beaccomplished either in a coal or oil fired boiler. The dewatered residueis high in fuel value and also contains sulfur capturing sodiumhydroxide and calcium carbonate. Applying the residue to coal as a dustcontrol has a positive impact on sulfur dioxide emissions from thefurnace or boiler.

The dewatered residue can also be used for dust control of fly ash. Flyash from coal burning operations is normally acidic, since the coal willcontain high levels of sulfur. By utilizing the residue for dustcontrol, the pH can be maintained closer to neutral.

As shown in the flow diagram the pulp slurry after the cleaningoperation and having a solids content of about 0.4% to 2.0% by weight ofsolids, is pumped to pulp washer 8, in which the dissolved solidscontained in the fiber supply, or generated through the pulpingoperation, are washed from the pulp. When dealing with recycled pulp,the washing operation can also remove dirt, fillers, or suspendedsolids, such as clay and calcium carbonate. The pulp washing can becarried out by a method as disclosed in U.S. Pat. No. 5,599,426, whichis incorporated herein by reference. As disclosed in that patentapplication, a very thin mat of pulp is supported between a pair ofporous endless belts and passed in a sinuous path over a series ofrolls. Wash water is impinged against opposite faces of the mat and thetension in the belts as they pass over the rolls creates a dewateringaction, so that the pulp is alternately showered with water and thendewatered by the belt tension, thereby removing fillers and dissolvedsolids from the pulp mat.

With the use of pulp produced from recycled material which contains ink,deinking and ink removal operations are required. The deinking can beaccomplished by pulping the recycled material with dispersant chemicals,such as a surfactant, that act to dissociate the ink from the fibers anddisperse the ink particles in the aqueous pulp. During the pulpingoperation, depending upon the nature of the ink, sodium silicate canalso be incorporated with the surfactant during the pulping. The sodiumsilicate provides alkalinity and aids in releasing and dispersing theink particles. In addition, hydrogen peroxide and chelating agents canbe utilized during pulping, which aid in solubilizing certain oils inthe ink, act as a brightening agent, and permit highly alkalineoperations without color reversion. Through this treatment duringpulping, the ink is dissociated from the cellulosic fibers.

Following the washing operation, the recycled pulp can then be subjectedto an ink removal operation. Ink removal can be achieved either by frothfloatation or dilution washing, which can include a side hill screenwasher, a gravity decker, a vacuum filter, an inclined screw extractor,or other types of washing. The publication "Beloit Corporation DeinkingManual" 2nd Ed., 1979, describes conventional ink removal processes.

After deinking the recycled pulp may have a dingy color due to a smallportion of residual ink, and thus the pulp is normally subjected tobleaching. In the bleaching operation, the pulp is thickened to aconsistency of about 15% solids, and bleached by adding a material, suchas hydrogen peroxide, ozone, or oxygen. The pulp is thickened in orderto reduce the amount of bleaching chemicals that are required.

The pulp then undergoes stock preparation, which normally consists ofrefining and dilution to provide the pulp in the headbox of thepapermaking machine 9, with a consistency of about 0.2% to 1.2% byweight solids.

In the papermaking process, the pulp as a thin web or layer is conveyedthrough the forming, press and dryer sections of the papermaking machine9 in a conventional manner to form the paper sheet.

In the papermaking operation, relatively pure water, having a minimummineral content is required for certain equipment, such as the showers,and vacuum pump sealing water. In accordance with the invention, thecondensate from storage tank 7 is used in the papermaking operation, asshown in the flow diagram. Depending on the water balance, make-up canalso be added to storage tank 7. The makeup water would normally bemunicipal water, and if the makeup water has a high mineral content, themakeup water can be added to the liquid effluent flowing between theclarifier 4 and the evaporator 6, so that the makeup water is thensubjected to the evaporation process.

Water recovered from the papermaking operation, which can consist ofwater recovered from the showers and pump seal water, as well as waterextracted from the wet paper web, is then recycled to the pulp washer 8and used as the wash water for washing the pulp. The water recoveredfrom the pulp washing operation, which consists of residual wash water,as well as water recovered by virtue of the increase in consistency ofthe pulp as it passes through the washing process, is then recycled tothe pulper 1 for use in producing the pulp slurry. In addition, aportion of the recirculated water as shown in the flow diagram, can berecycled to the pulp cleaner 2.

The invention maintains two distinct water systems. One water system ismaintained upstream or preceding the pulp washer 8 and includes thepulping vessel 1, pulp cleaner 3, clarifier 4, and dewatering unit 5.This water system contains a relatively high level of dissolved solids.The second water system located downstream of the pulp washer 8 is lowin dissolved solids. The wash water supplied to the countercurrent pulpwasher 8 is excess papermaking water and the water recovered from thepulp washing is utilized as make-up water in the pulping vessel.

In the pulp washer 8, the recycled pulp is subjected to counter-currentwashing. Normally recycled pulp is not washed in a counter-currentsystem. However, the counter-current washing acts to remove the majorityof the dissolved solids in the pulp through use of a relatively smallvolume of water, thus preventing the dissolved solids, as well ascolloidal material, from entering the paper machine water system, thusmaintaining a proper balance between the pulping water system and thepaper machine water system, so that there is no contaminated waterdischarge from the overall system.

The process of the invention provides a papermaking operation which haszero water discharge and the solids discharged from the process have aconsistency such that they can be either landfilled or incinerated.

The process also provides a higher fiber yield, and as there is no fiberloss in a water discharge as in conventional processes. In manyconventional papermaking operations, a substantial portion of the smallfibers or fines are lost in the liquid discharge of the operation.

As a further advantage, low pressure steam discharged from the dryersection of the papermaking machine is utilized as the heat source toevaporate the liquid effluent from the clarifier, and the condensatefrom the evaporation process serves as a heat source for the incomingwater to the papermaking machine. Thus, it is not necessary to directlyheat the incoming water as is required in a conventional papermakingoperation.

The process of the invention also enables brackish water having a highmineral content to be employed as make-up water by introducing themakeup water ahead of the evaporator, so that the makeup water is alsovaporized.

As the process has zero liquid discharge, the process utilizessubstantially smaller quantities of water than a conventionalpapermaking operation, and this has distinct advantages in arid regionsof the country where water is at a premium.

FIG. 2 illustrates a modified form of the invention. As in the firstembodiment, a recycled cellulosic pulp slurry is initially produced in apulping vessel 10 using conventional techniques. As previouslydescribed, the recycled cellulosic material can include used corrugatedcontainers, waste paper, white office waste, or the like, and normallycontains debris and solid contaminants. The recycled material also mayinclude a substantial quantity of water insoluble fillers, such as clayand calcium carbonate, as well as ink. In addition, the recycledcellulose may also contain residual water soluble materials emanatingfrom the original pulping or papermaking operation, such as sodium saltsof lignin, alum, starch and the like. In the pulping operation, pulpingchemicals, such as sodium hydroxide, are added to the pulp along withwater and conventional surfactants. Sodium silicate can also be used asa deinking aid and hydrogen peroxide as a brightener. During thepulping, the pulp is maintained at a temperature generally in the rangeof 120° F. to 160° F. and is subject to agitation or shearing to breakdown the fiber bundles.

As described with respect to the first embodiment, the pulp slurry,having a consistency generally in the range of 2% to 5% by weight ofsolids, is discharged from the pulping vessel to a pulp chest, notshown, and is then pumped from the pulp chest to a pulp cleaner 11. Inthe pulp cleaner, the pulp slurry is subjected to a conventionalcleaning procedure to remove suspended solids and particulate materialfrom the pulp. As described with respect to the first embodiment, largeand heavy debris is initially removed from the pulp slurry bycentrifugal cleaning as, for example, by using a liquid cyclone. Thepulp then undergoes a coarse screening to remove larger sizedcontaminants, followed by a finer secondary screening to remove smallersolids or contaminants.

As previously described, after the secondary screening, the pulp slurrythen undergoes several stages of forward cleaning forward followed byseveral stages of reverse cleaning. Following the reverse cleaning, thepulp slurry can be subjected to a conventional floatation clarificationto remove lighter weight contaminants.

Thus, the cleaning operation, after removal of the heavy debris,produces two aqueous flow streams, the first flow stream consistingprimarily of suspended cellulosic fibers that are substantially free ofsolid contaminants, along with water soluble dissolved solids. Inpractice, this pulp flow stream which is pumped to the pulp washer 12has a consistency generally in the range of about 0.5% to 3.0% solids.

The second or reject flow stream being discharged from the cleaningoperation consists of rejects from the coarse and fine screening,rejects from the forward and reverse cleaning operations, as well asrejects from the floatation clarification. Thus, this reject streamconsists primarily of suspended solids contaminants, along withdissolved solids. In practice, the reject stream may have a consistencyin the range of 2.0% to 2.5% solids.

The cleaned pulp stream from the pulp cleaner 11 is pulped to the pulpwasher 12, wherein the pulp is washed to remove dissolved solids thatoriginated from the recycled fibrous supply, or generated through thepulping operation. As described in the prior embodiment, the washingoperation can also remove residual dirt, fillers, or suspended solids,such as clay and calcium carbonate, when dealing with recycled pulp. Thewashing is preferably a countercurrent type and the washing can becarried out by a method as disclosed in U.S. Pat. No. 5,599,426, whichis incorporated herein by reference. As previously described, thewashed, recycled pulp can then be subjected to an ink removal operation,and then bleached, if desired, by a bleaching agent, such as hydrogenperoxide, ozone, or oxygen. The pulp then undergoes stock preparationconsisting of refining and dilution to provide the pulp in the head boxof the papermaking machine 13 with a consistency of about 0.2% to 1.2%by weight solids.

In the papermaking process, the pulp as a thin web or layer is conveyedthrough the forming, press and dryer sections of the papermaking machinein a conventional manner to form the paper sheet.

Makeup water, which normally would be municipal water, is supplied tothe papermaking machine 13, for use as showers, pump seal water, and thelike.

Water recovered from the papermaking operation, which can consist ofwater removed from the showers and pump seal water, as well as waterextracted from the wet paper web, is then recirculated to the pulpwasher 12 and used as the wash water for washing the pulp. As shown inFIG. 2, the water recovered from the pulp washing operation, as well aswater recovered by virtue of the increase in consistency of the pulp asit passes through the washing process, is then recycled to the pulpingvessel 10 for use in producing the pulp slurry. A portion of therecirculated water, as shown in FIG. 2, can also be recycled to the pulpcleaner 11.

The reject flow stream being discharged from the pulp cleaner 11normally has a solids content of about 2.0 to 2.5% by weight solids, andis delivered to a dewatering unit 14 where the material is dewatered,preferably by mechanical equipment, such as a compactor, screw press, orbelt press. The resulting dewatered sludge, which includes the solidcontaminants as well as a portion of the dissolved solids. has aconsistency generally of about 35% to 50% by weight solids and caneither be land filled or incinerated. The water extracted during thedewatering operation, which also contains dissolved solids, can berecycled to the pulp cleaner 11, as shown in FIG. 2.

One of the primary concerns in a zero liquid discharge papermakingoperation is the gradual buildup of dissolved solids in the water phase.A buildup of dissolved solids can adversely affect the physicalproperties of the paper being produced. With the system of theinvention, a substantial portion of the dissolved solids are removed inthe cake or sludge produced by the dewatering unit, so that a control isprovided for the buildup of dissolved solids in the water phase.

With the method of the invention, as shown in FIG. 2, additionaltreating operations, such as evaporation, reverse osmosis orultrafiltration to remove dissolved solids from the water phase are notrequired. However, the overall papermaking operation can be enhanced, ifdesired, by subjecting a portion of the water effluent from thedewatering unit 14 to a purification unit 15, as shown by the dashedlines in FIG. 2. The purification unit 15 serves to remove dissolvedsolids from the effluent and can constitute an evaporation, reverseosmosis or ultrafiltration system. The purified water from unit 15,which in the case of an evaporation system would be condensate, isdelivered to a water storage unit 16 where it is combined with make-upwater and fed as needed to the papermaking machine. The solid residuefrom unit 15 can be landfilled or incinerated, as shown in FIG. 2. Byutilizing the water purification unit 15, additional purified water ismade available for the papermaking operation and this, in turn,increases the available water for washing

If the make-up water is of low quality, i.e. has a high mineral content,the low quality water can be supplied to the purification unit 15, alongwith the liquid effluent from the dewatering unit 14, rather than beingsupplied to the water storage unit 16, as shown in FIG. 2. The purifiedwater from purification unit 15 is then delivered through water storageunit 16 to papermaking machine 13.

I claim:
 1. A method of papermaking have zero liquid discharge,comprising the steps of pulping recycled waste paper and recycled waterfrom a papermaking operation with pulping chemicals to form a pulpslurry, said recycled water containing dissolved water soluble solidsand said waste paper containing cellulosic fiber material, solidcontaminants, and water soluble solids, subjecting the pulp slurry to acleaning operation to produce a first flow stream containing saidcellulosic fiber material and dissolved solids and being substantiallyfree of said solid contaminants and a second flow stream consistingprimarily of suspended solid contaminants and dissolved solids, washingsaid first flow stream with water to remove residual dissolved solidsfrom said first flow stream and provide a washed pulp, utilizing thewashed pulp in a papermaking machine to provide a paper sheet, supplyingwater to the papermaking machine, recovering water from the papermakingmachine and utilizing the recovered water in the pulp washing operation,recovering water containing dissolved solids from the pulp washingoperation and utilizing the recovered water in the pulping operation,removing water from the second flow stream to produce a solid residueand a liquid residue containing dissolved solids, and utilizing theliquid residue in the pulp cleaning operation.
 2. The method of claim 1,and including the step of utilizing water containing dissolved solidsrecovered from the pulp washing operation in the pulp cleaningoperation.
 3. The method of claim 1, and including the step of removingwater from the second flow stream by mechanical operation.
 4. The methodof claim 1, and including the step of discharging the solid residue to alandfill operation.
 5. The method of claim 1, and including the step ofsubjecting the liquid residue to a purification treatment to produce asubstantially pure water phase and residual material, and incorporatingthe substantially pure water phase in the papermaking operation.
 6. Themethod of claim 5, and including the step of incorporating low qualitymake-up water with said liquid residue in the purification treatment toproduce the substantially pure water phase.
 7. The method of claim 5,wherein the purification treatment is evaporation.
 8. The method ofclaim 5, wherein the purification treatment is reverse osmosis.
 9. Themethod of claim 5, wherein the purification treatment isultrafiltration.
 10. A method of papermaking having zero liquiddischarge, comprising the steps of pulping recycled cellulosic fibermaterial with water and pulping chemicals to form a pulp slurry, saidcellulosic fiber material containing solid contaminants, subjecting thepulp slurry to a cleaning operation to provide a first flow streamcontaining a cleaned pulp and dissolved solids and being substantiallyfree of said solid contaminants and a second flow stream containingsuspended solid contaminants and dissolved solids, washing said firstflow stream with water to remove residual dissolved solids from saidfirst flow stream and provide a washed pulp, utilizing the washed pulpin a papermaking machine to provide a paper sheet, supplying water tothe papermaking machine, recovering water from the papermaking machineand utilizing the recovered water in the pulping operation, removingwater from the second flow stream to produce a compacted residuecontaining said solid contaminants and dissolved solids and a liquidresidue containing dissolved solids, and utilizing the liquid residue inthe pulp cleaning operation.